The radius is one of the two major bones found in the forearm, extending from the elbow to the wrist. It plays a significant role in the overall structure and movement of the arm, including forearm rotation, elbow flexibility, and wrist mobility. This bone, along with the ulna, forms the foundation for complex arm movements.
Location and Structure
The radius is located in the forearm, on the lateral (thumb) side when the arm is in the anatomical position. It is a long bone, thicker and shorter than the ulna. The radius widens considerably at its lower end where it connects to the wrist, while its upper end is smaller where it meets the upper arm bone.
At its proximal end, the radius features a cylindrical radial head that articulates with the capitulum of the humerus (upper arm bone) at the elbow joint. Below the head, the radial tuberosity serves as an attachment point for certain muscles. The shaft of the radius is slightly curved and provides support to the forearm. Distally, the radius broadens significantly, forming the top of the wrist joint and articulating with two carpal bones: the scaphoid and lunate. The lateral margin of the distal radius forms the radial styloid process near the wrist.
Role in Movement
The radius is instrumental in the movements of the forearm and wrist, allowing for flexibility. One of its primary functions is enabling pronation and supination: rotational movements that turn the palm downward (pronation) and upward (supination). This rotation occurs as the radius pivots around the ulna at both the elbow and wrist joints. The radial head rotates within a ligamentous ring at the elbow, while the distal end of the radius crosses over the ulna during pronation.
The radius also contributes to various wrist movements. It forms the main articulation at the wrist joint with the carpal bones. This articulation facilitates movements such as wrist flexion (bending the hand forward), extension (bending the hand backward), and radial deviation (tilting the hand towards the thumb side).
Common Conditions
The radius is susceptible to injuries, with fractures being common. Distal radius fractures, occurring near the wrist, are frequent, often resulting from a fall onto an outstretched hand (FOOSH). These fractures are common in both younger individuals (due to high-energy trauma) and older adults (especially those with osteoporosis). A common type is a Colles’ fracture, where the broken fragment of the radius tilts backward, sometimes creating a “dinner fork” deformity in the wrist.
Another type of distal radius fracture is a Smith’s fracture, which is less common and involves the broken fragment shifting forward. Symptoms of distal radius fractures typically include immediate pain, swelling, tenderness, and sometimes a visible deformity of the wrist. Diagnosis usually involves X-rays, and treatment can range from immobilization with a cast for stable fractures to surgical intervention for more complex or displaced breaks.
Radial head fractures, occurring at the elbow end of the radius, are also common, often caused by falls onto an outstretched arm. These fractures can cause pain on the outside of the elbow, swelling, and difficulty rotating the forearm. Treatment depends on the severity, from a sling for minor fractures to surgery for multiple bone fragments. Less common, but possible, are stress fractures of the radius, which result from repetitive forces without adequate recovery, sometimes seen in athletes.